5 Department of Geography and Regional Research, University of Vienna, AT-1090 Vienna, Austria.

Abstract

Extracts of soil are used to provide estimates of plant-available nitrogen sources such as nitrate, ammonium and amino acids (low molecular weight nitrogen, LMW-N). Soil extracts are a blunt tool; they introduce a number of inaccuracies through soil disturbance, and do not indicate the rapidity of N pool turnover. Microdialysis is used predominantly in neuroscience but was recently introduced in soil research. Small in situ probes cause minimal disturbance, and passive diffusion of solutes across a semi-permeable membrane allows dialysate to be collected over time allowing study of nutrient flux dynamics. This sampling mode is functionally similar to plant roots, and may provide a good estimate of the N forms available to roots. We used microdialysis to quantify induced diffusive fluxes of LMW-N in a subtropical agricultural soil under three fertiliser regimes. Shifts in LMW-N fluxes were detected over time, suggesting the formation of depletion zones around the probe surface similar to those associated with roots. A pronounced difference was observed between results from microdialysis and soil extracts. In dialysate, amino acids contributed up to 70% of LMW-N in unfertilised soil and 5-20% in fertilised soils. In contrast, amino acids were a minor constituent in soil extracts, highlighting that soil extracts underestimate amino acid availability in soils. Modelling plant N uptake based on soil N fluxes and root uptake kinetics, we show that use of inorganic N in fertilised soil was constrained by the root’s uptake. In contrast, fluxes of amino acids and the root’s uptake capacity were closely matched.

Yields are primarily dependent on the input of nitrogen. Since fertilization poses serious environmental concerns, better performing maize cultivars with reduced nitrogen inputs are required. The aim of this work is to develop efficient phenotyping procedures to assess differences in nitrogen uptake and nitrogen use efficiency at the reproductive stage of maize cultivars supplied with different nitrogen doses. Both, active and passive sensor systems were tested to assess such traits and the spectral information was related to destructively assessed parameters of the aboveground biomass and nitrogen uptake at flowering, kernel dough stage and at grain harvest. In general, spectral indices obtained from the hyperspectral sensor were better related to the biomass or the nitrogen uptake of maize cultivars than those obtained from active sensors. Optimized hyperspectral indices were closest related to the leaf dry biomass with R2-values of 0.55 and to the Nitrogen Nutrition Index with R2 = 0.77 at the kernel dough stage. PLSR models were calculated for two experimental years, using data from 2015 as calibration data set and from 2014 as validation data set and allowed to predict leaf nitrogen uptake with R2 = 0.70 and RMSE of 10.8 kg N ha-1. The phenotyping platform PhenoTrac 4 developed by TUM allowed to obtain enhanced information of the complex traits nitrogen uptake and nitrogen use efficiency. In combination with improved algorithms using both optimized indices as well as PLSR models promising results were obtained to further narrow the gap existing between genomics and phenomics.

The remarkable influence of organic inputs, such as manure and compost, on d15N values of growing plants suggests the possible use of 15N natural abundance as a tracer of N. Thus, using 15N natural abundance might be possible to estimate compost-N recovery as an alternative method to the use of 15N-enriched materials. The objective of this study was to verify the feasibility of using d15N value to estimate compost-N recovery by plants. Head lettuce, carrots and broccoli were cultivated (randomized blocks) in sequence under increasing levels (0 to 2.5 kg/m², dry matter) of compost application. Pearson correlations were significant and positive between plants d15N and yield (except lettuce, no yield response). A new equation for estimating compost-N recovery by plants was proposed using differences in d15N-plant with and without compost application. The compost-N recoveries were 2-8 % for lettuce, 4-9 % for carrots, and 9-18 % for broccoli. Unrealistic estimates were disregarded and assigned primarily to non-representative sampling because of intra-plant d15N variations. This study showed the theoretical and experimental basis of using d15N values to estimate compost-N recovery by test plants.

Ammonia (NH3) is an important atmospheric base which can contribute to eutrophication, acidification of ecosystems and the formation of secondary aerosols. In order to monitor potential policy driven reductions and provide information on NH3 emissions, transport and deposition, active sampling is required. There are, however, many challenges in measuring NH3 as it is a ‘sticky’ reactive molecule which is readily soluble and rapidly interacts with other trace gases to form secondary inorganic aerosols. When undertaking ambient NH3 measurements, inlet set up, use of filters, reliable calibration standards and potential chemical interferences in the analytical technique thus need particular attention. Due to these complexities of NH3 active measurements, a metrology-centred project on “Metrology for Ammonia in Ambient Air” (MetNH3) funded by the European Metrological Research Programme (EMRP), was started in 2014, with the aim of developing traceable NH3 monitoring methods and NH3 certified reference material, which will be applicable under field conditions. In this project a commercial cavity ring down instrument is being characterised and further developed to be the traceable method of choice, alongside the development of an open path absorption spectrometer.

The following study presents the first results of a field intercomparison of different NH3 instrumentation verified against traceable methods developed and characterised as part of the MetNH3 project, held in South East Scotland. In addition, the applicability of dynamic calibration systems, under field conditions which have developed within the project is assessed. The overall objective of this study will be to establish recommendations for ambient NH3 measurements.